Background Although many reports have shown that preoperative dementia affects surgical prognosis, it is unclear whether the dementia based on Mini-Mental State Examination (MMSE) affect hospital mortality in old old patients undergoing elective gastrointestinal surgery. Aims This study aims to investigate whether preoperative dementia might affect the outcomes of old old patients undergoing elective gastrointestinal surgery by evaluating with the MMSE. Methods All patients aged >= 75 years who undergoing elective gastrointestinal surgery form January 2015 to December 2021 in a Chinese tertiary hospital were retrospectively analyzed. Their preoperative cognitive status was evaluated using the MMSE, and analysis was performed to compare the patients with MMSE score < 24 (dementia group) or MMSE score 24-30 (non-dementia group). Risk factors for hospital mortality were explored using multivariate logistic regression analysis. Results 980 patients were rolled in the study, and 102 (10.4%) patients were in the dementia group. ICU and hospital stay were longer in the dementia group. Regarding the postoperative complications, the incidence of cerebral infarction (P = 0.014), delirium (P = 0.019), and pulmonary infection (P = 0.017) was more frequent in dementia group. Hospital mortality was 11% in the dementia group and 3% in the non-dementia group (P = 0.009). Multivariate logistic regression analysis revealed that dementia (P = 0.0135), preoperative lower albumin (P = 0.0018) and malignancy (P = 0.0212) were independent risk factors for hospital mortality. Conclusions Among old old patients undergoing elective gastrointestinal surgery, hospital mortality was increased significantly for patients with dementia evaluating with the MMSE.